Flaw detection method using a liquid solvent developer



May 18, 1965 J. R. ALBURGER 3,184,596

FLAW DETECTION METHOD USING A LIQUID SOLVENT DEVELOPER Filed 00'0. l0.1961 United States Patent O 3,184,596 FLAW DETECHUN ME'EHGD EISEN@ AHQUED SLVENT DEVELPER .llames R. Albergen 59h37 Hilliard Ave., LaCanada, Calif. Filed @et 10, 1961, Ser. No. 147,391 S Claims. (Cl. d-71) This invention relates to a method of and system for detectingcertain types of particularly small daws, such as shrink cracks, stresscracks, intercrystalline cracks, or corrosion microleaks, cold solderjoints, etc., with penetrant tracers not normally visible under eithernormal lighting or under ultraviolet light.

In my copending application, Serial No. 82,374, filed January 13, 1961,now abandoned, a method of and system for detecting and measuring thesensitivity values of different fluorescent penetrant inspection tracersare disclosed .and claimed, .the system also measuring the absolutesensitivity ratings of such tracers. The measurement of sensitivity ismade .by measuring the spot diameter at the color threshold of thetracer.

It has been found that where a fluorescent or visible color dye is usedin a liquid or resinous material as a tracer indicator, there is acharacteristic dimensional sensivity whereby, at a certain critical filmthickness, there exists a threshold below which the fluorescence orcolor substantially disappears. There is always a range of filmthicknesses over which there occurs a transition from noniluorescence tofluorescence, or no-color to color. Sensitivity ratings for given tracermaterials in terms of Sensitivity Index (IS) values, define thedimensional thresholds at which fluorescence or color disappears.

Many important applications of visible color or fluorescent tracersinvolve film thicknesses which may be close to or even below thethreshold condition where fluorescence or color disappears. In extremelyminute surface discontinuities, such as mentioned above, the film oftracer material may be on the order of l0*5 cm. or less, and at suchdimensional magnitudes it is often impossible to detect any color orfluorescence in the tracer. This invention contemplates the developmentof fluorescent or visible color indications to a .point of practicalseeability or detectability even when the tracer film or entrapment isdimensionally smaller than that corresponding to its limit ofsensitivity.

The invention also contemplates the improvement in performance offluorescent sensitizers and tracers well beyond any previouslyattainable level. ln this manner, it has become possible to extend thesensitivity o-f a tracer material even farther into the ultra-small;that is, flaws having dimensions on the order of a few millimicrons orless.

The principal object of the invention, therefore, is to ta- .cilitatethe detection of extremely small flaws.

Another object of the invention is to provide an improved method of andsystem for treating materials in the inspection for microscopic orsub-microscopic surface flaws.

A vfurther object of the invention is to provide a developing method anda developer to increase the fluorescence or visibility of tracers.

A better und-erstanding of this invention may be had from the followingdetailed description when read in conjunction with the accompanyingdrawings, in which:

FIG. 1 indicates a series of flaws illustrating the invention; and

FIG. 2 is a graph illustrating the operation of the invention.

In my copending application, Serial No. 68,267, now Patent No.3,107,298, filed November 9, 1960, it is disclosed that the threshold offluorescent incidence for a ldgb Patented May 18, 1955 lCC given dyefollows approximately a rectilinear line drawn on single logarithmicgraph paper when the dimensional threshold is plotted against dyeconcentration. It is also disclosed that this applies to bothfluorescent and visible color dyes. Such a graph is shown in FIG. 2. Thegraph of FIG. 2 .along with the reference rectilinear line, and thescales of spot diameter, film thickness and concentration, have becomestandard in industry. The shapes of the sensitivity curves A and B shownin FIG. 2 indicate that an increase in film thickness with thecorresponding decrease in concentration yields an increase influorescence o-r color response. This means that if the thin film oftracer material has a dimension such that its fluorescence or colorresponse is just below the threshold of visibility, the addition of asuitable solvent or resinous material to increase the film thicknesswill increase the `color or the fluorescent response to rise above thethreshold of visibility. Also, a tracer film which is faintly vis-iblecan be rendered more distinctly visible merely by increasing itseffective film thickness even though the concentration of the dye isdecreased in the process.

To illustrate the action of my developer process and method, I may havea certain tracer material corresponding to the No. 7 Sensitizer, whichhas a .characteristic shown by curve B and which has a concentrationvalue of 60 on the lchart of FIG. 2. Assume that a minute entrapment ofthis tracer has a film thickness of millimicrons, which is below thecritical threshold of fluorescent response, and hence no fluorescence:can be seen. The operating point of the tracer is indicate-d by 13 onthe chart. lf I now apply a liquid developer to this tracer entrapmentso that its film thickness is increased by a factor of 3 and itsconcentration simultaneously reduced by a factor of 3, then theoperating point on the tracer film is moved to the left and up to point14 on the chart. This new operating point is now above the threshold offluorescent response so that a clearly visible fluorescent indicationcan be seen, where none could be seen prior t-o the application of thedeveloper.

Conventional developers consist essentially of a slurry of fine powderedparticles. For extremely small flaws, however, these powder-typedevelopers are not satisfactory 4because the powder particles obscurethe minute flaw indications or interfere with an examination of theshape of the flaw. The developer described hereinafter does not havethese disadvantages and is a solvent which is compatible with theparticular dye or senstizer material employed in the original tracer. Mysolvent has a relatively high viscosity and low solvency so that dyetracers reach out into the vapplied solvent film over a period of time.To increase its viscosity, a thickening agent is used, such as aresinous material soluble in the solvent. With this developer, thesolvent and resinous thickener is compatible with the tracer dye andsensitizer so that as the dye bleeds into the developer film, it retainsits color or fluorescence.

Referring again to the drawings, and particularly to FIG. l, it is notedthat the aw 5 is of small dimensions compared with the flaw d. The fiaw5 will exude a small film 7 over a period of time, while the flaw t5will exude the film 3 over the flaw. Flaw 5' may be of such size that itis not visible or will not give a visible indication under black light.However, a small flaw 1t), comparable in size to the flaw S, has itseXuded film lll but is covered with a developer 12 which causes it tobecome visible in accordance with the chart in FIG. 2 for the reasondescribed above. The particular developers and dyes and penetranttracers used will now be given by way of examples.

Example 7 In the detection of minute porosities or fracture cracks inunglazed ceramic parts, the parts are immersed in a red, oily penetrantwhich will employ a solvent system which is insoluble in water. Theceramic parts are allowed to dwell in the dye penetrant for ten minutes,after which they are drained` and immersed in an emulsifier for a periodof one minute. The parts are then washed thoroughly in water to removeall traces of surface penetrant. At this point, minute entrapments inporous pits and cracks may be invisible because their dimensions areless than the threshold of visibility. The parts are now immersed in awater solution of methyl cellulose of about one percent concentrationand allowed to drain and dry. This applies an extremely thin film ofmethyl cellulose to the surface of the parts, thus providing a barrierto slow down the rate at which the entrapped dyes can diffuse. After theparts are dried, they are sprayed with a thin coating of an acrylicplastic resin which is in a ketone solvent. Within a few seconds orminutes, while the resin film is drying, traces of dye are drawn out ofthe entrapments and form distinctly visible red indications. In thisexample, a short oil alkyd resin, dissolved in an alcohol solvent, maybe substituted for the acrylic plastic developer.

Example Z 'Y parts to be inspected are thoroughly cleaned and are al- Ylowed to dwell for from ten to fifteen minutes in an lultra-sensitivefluorescent penetrant. The parts are then drained, emulsified for aboutone and one-half minutes, and washed thoroughly. At this point, thetracer entrap- 'ments smaller than 42 millimicrons are invisible. Theparts are then dipped in a semi-viscous mixture of water, methylcellulose thickener, and ethoxy nonyl phenol and are allowed to dry. Inthe wet condition, the water inhibits the tendency of thewater-insoluble tracer to eXude from the entrapments. As the parts dry,ethoxy nonyl phenol begins to act on the tracer, drawing the dye outinto itself to form a relatively thick film which yields a visiblefluoroescence under black ,light inspection. In this example, lowvolatility glycol ether or water soluble surfactants, suc-h aspolyethylene sorbitan monolaurate may be substituted for the ethoxynonyl phenol.

Example 3 In the process for inspecting solder connections on anelectronic printed circuit board, a iiuorescent tracer is incorporatedin `a resin type iiuX which is'used in the soldering. The sensitivityindex of the solder flux may be as high as is practical to make it, suchas 100 or more. After soldering is complete, the board assembly iswashed thoroughly in isopropyl alcohol so as to remove any solder iiuxwhich remains on the surface and the board is dried. At this point,minute linx entrapments, which may be smaller than -5 in thickness, willbe below the thres- V hold of visibility. The circuit board is thensprayed with VIn testing for leaks in a water pipe or valve system, awater soluble tracer dye combination is incorporated in a volume ofwater inside the object, the sensitizer and color former providing anadequate level of sensitivity. On'the outside of the Valve, tank, orpipeline, a developer is applied consisting of a viscous solution ofwater and a thickener, such as methyl cellulose. As minute traces oftracer liquid leak through small openings, porosities or leakage paths,they diffuse into the film of viscous developer and become readilyvisible due to the augmentation of effective tracer film thickness.

4 I claim: 1. The method of detecting surface aws in a part comprisingthe steps of,

(a) immersing said part in a dyed, oily water-insoluble penetrant; (b)draining said part and immersing in an emulsifier;

(c) washing said part in water to Vremove all surface Y 2. The method ofdetecting surface flaws in a part comprising the steps of,

(a) immersing said part inr an ultra-sensitive fluorescent penetrant;(b) draining and emulsifying said part; (c) Washing said partthoroughly; and (d) immersing said part in polyethylene sorbitanmonolaurate and then drying, to bring the operating point of the dye insaid fiuorescent penetrant from below the threshold of visibility to anoperating point above the threshold of visibility. 3. The method ofdetecting surface flaws in solder connections comprising the steps of,

(a) incorporating a fluorescent tracer in a resin-type soldering flux;(b) washing said solder connections in a solvent adapted to dissolvesaid soldering iux; (c) drying said solder connections; and (d) sprayingsaid solder connections with a thin film of fast-drying plastic resinsolution, to bring the operating point of the dye in said fluorescentpenetrant from below the threshold of visibility to an operating pointabove the threshold of visibility. 4. In the non-destructive testing forflaws in surface, the step of applying over said surface which containsin said flaws entrapments of dyed flaw-penetrating material, a liquidsolvent developer adapted to dissolve said dyed haw-penetrating materialto simultaneously increase the thickness of any exuded film from saidiiaws and dilute the dye concentration thereof to bring the operatingpoint of the dye in said penetrating material from below the Y thresholdof visibility to an operating point above the threshold of visibility. Y

5. In the nondestructive testing for surface aws, the step of applyingover said surface thereof which contains in said flaws entrapments ofdyed flaw-penetrating material, a liquid developer adapted tosimultaneously increase the thicknessV of any exudedV film from saidflaws and dilute the dye concentration thereof to bring the operatingpoint of the dye in said penetrating material from below Vthe thresholdof visibility to an operating point above the threshold of visibility,said liquid developer being a plastic resin adapted to dissolve the dyeyin said penetrating material.

6. The method of detecting surface flawsin a partcomprising the stepsof, l

(a) Immersing said part in a dyed oily water-insoluble penetrant; (b)VDraining said part and immersing in an emulsifier; (c) Washing saidpart in water to remove all surface traces of said penetrant; Y Y Y (d)Draining and drying said part; and (e) Spraying said part with a thincoating.V of plastic 7. An inspection method for detecting a surfacetiaw material eXuded from said iiaw, and applying to the sur- Y Y faceof said part a liquid solventdeveloperY adapted to dissolve said dyedHaw-penetrating material to simultaneously increase the effectivethickness of said lm of tracer indication and decrease the concentrationof said dye in said film to bring the operating point of saidpenetrating material from below the threshold of visibility to anoperating point above the threshold of Visibility.

8. The method of detecting surface flaws in a part comprising the stepsof,

(a) iinmersing said part in an ultra-sensitive fluorescent penetrant;(b) draining and emulsifying said part; (c) Washing said partthoroughly; and (d) immersing said part in a semi-viscous mixture ofwater, water-soluble thickener and ethoxy nonyl phenol and then drying,to bring the operating point of the dye in said fluorescent penetrantfrom below the threshold of visibility to an operating point above thethreshold of visibility.

References Cited by the Examiner UNITED STATES PATENTS 2,478,951 s/49 r2,663,692 12/53 d 2,667,070 1/54 2,676,487 4/54 2,806,959 9/57 2,953,5309/60 3,027,75/1r 4/62 10 3,028,338 4/62 3,117,227 1/64 Stokely et al252-408 Corso et al 252-408 Sockrnan et al, 73-104 Clarke et al 73--104De Forest Z50-7l Switzer 250--71 Alquist 7340.7 Parker 250-71 Pollack250-71 RALPH G. NILSON, Primary Examiner. 15 JULIUS GREENWALD, JAMES W.LAWRENCE,

Examiners.

2. THE METHOD OF DETECTING SURFACE FLAWS IN A PART COMPRISING THE STEPSOF, (A) IMMERSING SAID PART IN AN ULTRA-SENSITIVE FLUROESCENT PENETRANT;(B) DRAWING AND EMULSIFYING SAID PART; (C) WASHING SAID PART THOROUGHLY;AND (D) IMMERSING SAID PART IN POLYETHYLENE SORBITAN MONOLAURATE ANDTHEN DRYING, TO BRING THE OPERATING POINT OF THE DYE IN SAID FLUORESCENTPENETRANT FROM BELOW THE THRESHOLD OF VISIBILITY TO AN OPERATING POINTABOVE THE THRESHOLD OF VISIBILITY.